您的代码速度慢有两个主要原因，您使用Where-Object执行线性查找，而Where-Object本身速度很慢（这是PowerShell中过滤集合的最慢技术），但除此之外，您在每次循环迭代中执行2次线性查找，而本可以只执行一次：

# lookup just once:
$lookUp = $directoryTable | Where-Object { $_.employeeid -eq $changeRecordLine.employeeID }
# then Name and DN are available to you:
$lookUp.Name
$lookUp.DistinguishedName

What you should use instead of a linear lookup is a structure meant specifically for fast lookups:

$tableObj = Import-Csv ".\employeeIDsAndAttributes.csv"
$map = @{}

# `name, samaccountname, distinguishedname` are already default properties
# no need to include them in `-Properties`
foreach($user in Get-ADUser -Filter "EmployeeId -like '*'" -Properties employeeid) {
    $map[$user.employeeid] = $user
}

foreach($changeRecordLine in $tableObj) {
    $value = $map[$changeRecordLine.employeeid]
    $props = $changeRecordLine.PSObject.Properties

    $parentDN = try {
        $changeRecordLine.DN.SubString($changeRecordLine.Name.Length + 4)
    }
    catch { }

    # `Add-Member` adds overhead even though this is the least of your code's issues,
    # adding `NoteProperties` to your objects by accessing
    # the object's PSObject Properties and adding them manually is faster
    $props.Add([psnoteproperty]::new('Name', $value.Name))
    $props.Add([psnoteproperty]::new('DN', $value.DistinguishedName))
    $props.Add([psnoteproperty]::new('ParentDN', $parentDN))
}